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Asteroid impact avoidance comprises a number of methods by which near-Earth objects (NEO) could be diverted, preventing destructive impact events. A sufficiently large impact by an asteroid or other NEOs would cause, depending on its impact location, massive tsunamis, multiple firestorms and an impact winter caused by the sunlight blocking effect of placing large quantities of pulverized rock dust, and other debris, into the stratosphere. A collision between the Earth and an approximately 10-kilometre-wide object 66 million years ago is thought to have produced the Chicxulub Crater and the Cretaceous–Paleogene extinction event, widely held responsible for the extinction of the dinosaurs. While the chances of a major collision are not great in the near term, there is a high probability that one will happen eventually unless defensive actions are taken. Recent astronomical events—such as the Shoemaker-Levy 9 impacts on Jupiter and the 2013 Chelyabinsk meteor along with the growing number of objects on the Sentry Risk Table—have drawn renewed attention to such threats. == Deflection efforts == Most deflection efforts for a large object require from a year to decades of warning, allowing time to prepare and carry out a collision avoidance project, as no known planetary defense hardware has already been developed. It has been estimated that a velocity change of just (where t is the number of years until potential impact) is needed to successfully deflect a body on a direct collision trajectory. In addition, under certain circumstances, much smaller velocity changes are needed.〔S.-Y. Park and I. M. Ross, "Two-Body Optimization for Deflecting Earth-Crossing Asteroids," ''Journal of Guidance, Control and Dynamics,'' Vol. 22, No.3, 1999, pp.415–420.〕 For example, it was estimated there was a high chance of 99942 Apophis swinging by Earth in 2029 with a 10−4 probability of passing through a 'keyhole' and returning on an impact trajectory in 2035 or 2036. It was then determined that a deflection from this potential return trajectory, several years before the swing-by, could be achieved with a velocity change on the order of 10−6 ms−1.〔Lu, Edward T. and Stanley G. Love. (A Gravitational Tractor for Towing Asteroids ), NASA, Johnson Space Center, submitted to arxiv.org September 20, 2005. ((PDF document )).〕 An impact by a asteroid on the Earth has historically caused an extinction-level event due to catastrophic damage to the biosphere. There is also the threat from comets coming into the inner Solar System. The impact speed of a long-period comet would likely be several times greater than that of a near-Earth asteroid, making its impact much more destructive; in addition, the warning time is unlikely to be more than a few months.〔(【引用サイトリンク】 title=Report of the Task Force on potentially hazardous Near Earth Objects ), p. 12.〕 Impacts from objects as small as in diameter, which are far more common, are historically extremely destructive regionally (see Barringer crater). Finding out the material composition of the object is also helpful before deciding which strategy is appropriate. Missions like the 2005 ''Deep Impact'' probe have provided valuable information on what to expect. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Asteroid impact avoidance」の詳細全文を読む スポンサード リンク
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